Note to any aspiring haters out there: Rihanna is not one to be trifled with.

First, she flogged members of the media in her video for her [article id="1657050"]current single "S&M,"[/article] then she went all scorched-earth on censors in the U.K., who changed the title of that song to [article id="1657392"]"Come On"[/article] without her knowledge and earned a fierce rebuke for their efforts. (Rihanna personally vowed to have the title changed back ... and, less than a week later, it was.)

And on Friday, she turned her wrath upon Ciara, after the singer dared dis RiRi during an appearance on E!'s "Fashion Police" with Joan Rivers.

"I ran into [Rihanna] recently at a party. She wasn't the nicest," Ciara told Rivers. "It's crazy, because I've always loved and respected what she's done in fashion. It wasn't the most pleasant run-in."

Of course, things were about to get a lot less pleasant. Soon after Ciara dared speak her name, Rihanna took to her Twitter account to strike back brand-new rival ... in less than 140 characters.

"My bad, Ci, did I forget to tip you," Rihanna wrote, including the hashtag #howrudeofme for extra spite.

Of course, that earned a response from Ciara, who wrote, "Trust me, Rihanna, you don't want to see me on or off the stage."

Rihanna replied first with a taunt ("U gangsta huh? Haaa" (link and then, after thinking about it for a minute, went for the jugular, making fun of [article id="1658536"]Ciara's recent sales woes[/article] by writing, "Good luck with booking that stage you speak of."

Ciara tried to laugh off the slight, while Rihanna, perhaps feeling a twinge of remorse after vivisecting her latest foe, posted an apology: "Ciara baby, I love you girl! You hurt my feelings real bad on TV! I'm heartbroken! That's why I retaliated this way! So sorry!"

Ciara accepted RiRi's apology and suggested the two work out their problems "in person." Rihanna has yet to respond.